I'm glad I stumbled across this, as I haven't read it since I was a child, although as I was reading it this time I could still vaguely remember bits of it. First published just over a hundred years ago this book has entertained countless adults and children since then.
When siblings, Gerald, James and Kathleen can't go home form their respective boarding schools in the holidays, due to illness at home, it is decided that they will stay at Kathleen's school, and be looked over by a servant and the French Mistress. Thus the adventures of this intrepid trio soon begin. Deciding to take a picnic they stumble across what they decide is an enchanted castle, and meet the princess of the said castle, or Mabel as it becomes apparent after some time. With Mabel claiming that a ring is magic, much to everyone's surprise it does appear to be so, and thus their adventures get under way, but also the grounds of the building also seem to be enchanted.
Children will love this, when they are younger to have it read to, as they are older, to read themselves, and then again as adults, and then starting the whole process off again if they have children. This tale is simply timeless and can appeal to all ages, as when you are older it reminds you of your own imagination as a child. Full of magic, and some romance, there isn't really anything that you can find wrong with it.